1. Field of the Invention
The invention relates to the production of ammonia synthesis gas. More particularly, it relates to an improved process and apparatus, incorporating pressure swing adsorption technology, for producing such ammonia synthesis gas.
2 Description of the Prior Art
Ammonia synthesis gas production is conventionally based on the steam reforming of natural gas or naphtha, followed by secondary reforming with air. Because of the dramatic increase in energy costs in recent years, efforts have been made to achieve processing and energy savings in such operations. The potential for process simplification by the use of pressure swing adsorption (PSA) technology for hydrogen purification has been recognized in the art. This has particular commercial significance since ammonia production represents the largest hydrogen consumer of all chemical processing industry. In contrast to other approaches, including combinations of wash systems, methanation and the like, the PSA approach enables the essentially complete removal of all contaminants and inerts from the hydrogen-containing gas in a single purification.
While the steam reforming of natural gas or naphtha is generally preferred for feedstock conversion in ammonia production, it is also possible to subject feedstocks to partial oxidation conversion, and there has been some trend toward basing commercial plants on the use of oxygen for the partial oxidation of heavier petroleum fractions and coal. In ammonia production operations based either on steam reforming or on partial oxidation operations, an air separation system, or a nitrogen plant, has been incorporated as a necessary element of the overall system. Thus, the oxygen from such a system has been employed for the partial oxidation of hydrocarbon feedstocks. The nitrogen recovered from such a system has been mixed with the purified hydrogen discharged from the PSA system to form the desired ammonia synthesis gas.
While the use of PSA technology in the production of ammonia synthesis gas has been found advantageous, there is a continuing need in the art for the development of improvements to offset the high cost of energy. One response to this need has been the proposed use of external source nitrogen as a purge gas in the purification of hydrogen by pressure swing adsorption techniques for use in ammonia production operations. By such means, nitrogen is conveniently recovered from the PSA system, together with purified hydrogen, at the higher adsorption pressure employed in the PSA system. By employing the nitrogen at an elevated pressure as such purge gas, the purge gas effluent removed from the PSA system can be used for desirable power generation purposes. The power thus generated can be used to compress air passing to an air separation or nitrogen plant, with the nitrogen recovered therefrom being available for convenient use as the elevated pressure, external source nitrogen purge gas for the PSA system. The recovered oxygen can be used in a secondary reforming unit, if employed, or in a partial oxidation unit, if so employed, for the generation of hydrogen. This approach will be seen, therefore, to enable a highly desirable, overall integration to be achieved with respect to the hydrogen generation, PSA-hydrogen purification, ammonia systhesis and air separation portions of the overall system for converting hydrocarbon feedstocks into ammonia synthesis gas. Such integration enables ammonia synthesis gas and product ammonia to be produced at lower energy costs than is possible using other technologies presently available in the art. The use of elevated pressure nitrogen purge gas also enables the design of the PSA system to be simplified as a result of the lower pressure drop pertaining across the adsorbent beds employed therein. Despite such highly desired benefits, there remains in the art a desire and need for further improvements and enhancement of the overall system and process for the production of ammonia synthesis gas by means of pressure swing adsorption technology. Indeed, such continuing improvements are necessary if ammonia synthesis gas and ammonia are to be available to economical costs, despite the high cost of energy, so as to satisfy the growing requirements of industrial societies throughout the world.
It is an object of the invention, therefore, to provide an improved process and apparatus for the production of ammonia synthesis gas.
It is another object of the invention to provide a process and apparatus for enhancing the use of pressure swing adsorption technology in the production of ammonia synthesis gas.
It is a further object to provide a process and apparatus capable of reducing the costs associated with the overall system for converting hydrocarbon feedstocks to desired ammonia synthesis gas.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.